Electrical cables for signal transmission are mostly provided with a tube-shaped shield surrounding the signal conductors of the cable. The shield consists of an electrically conductive material in order to prevent electromagnetic interference fields from affecting the signal conductors and to suppress electromagnetic radiation from the cable, which might disturb adjacent cables or units. Such shields primarily consist of a metal braiding surrounding an insulating sheath which covers the signal conductor. The braiding is covered by an insulating outer jacket of the cable.
Cables of this type are frequently terminated by connectors, in particular plug connectors in order to connect the signal conductor with other cables or units. In order to avoid electromagnetic interference in the area of these connectors, the latter are usually surrounded by a metal housing which is electrically connected to the cable shield. Frequently, backshells are provided. These are connected with the cable shield on a side at which the cable is inserted and which are attached to the metal housing of the connector at the opposite side, for example, by screw connections. The function of these backshells is twofold: first, they serve as a strain relief for the cable; second, they are used to transmit the electrical potential of the cable shield to the potential of the metal housing.
For this reason, known backshells are provided with a sleeve part surrounded by a shield, whereas the remaining part of the cable is guided through the sleeve into the backshell. In order to apply the shield to the outer circumference of the sleeve part, the shield braiding has to be opened so that it fits into the sleeve part. The area of the opened shield which had been slipped over the sleeve part is then crimped onto the sleeve part, where it is attached by means of a tape or a clamp, or it is shrunk under a magnetic field. In the latter case, the connection is irreversible and does not allow for any repairs.
When the braiding of the shield is opened for slipping it over the sleeve, the shield becomes electromagnetically permeable, so that the entire system containing said connector will become susceptible to electromagnetic interference.
A device for screwing a coaxial cable end to an amplifier housing is known from DE 83 29 128 U1, wherein the shield is only indirectly connected to the sleeve by a contact cage. The contact cage is located in a radial gap between the shield and an enlarged inner diameter of the sleeve. One axial end of the contact cage reaches up to the corresponding axial end of the sleeve.
In that region of frequencies in which high-frequency cables are mainly utilized, namely those beginning at approximately 1 MHz, a contact cage behaves like a slot antenna. This means, it emits high-frequency radiation and absorbs incoming high-frequency radiation. For this reason, the end area of the cable jacket covering the shield which is located adjacent to the sleeve and the contact cage cannot prevent interference by emitted or absorbed radiation. In order to assure a sufficient electrical contact between the sleeve and the shield, taking into account expected production tolerances, the contact cage must have a relatively strong spring force. In case of cables of particularly good high-frequency characteristics, the dielectric between the shield and the inner conductor often consists of a relatively soft material, such as expanded microporous polytetrafluoroethylene (PTFE). This soft material is radially compressed towards the inside by the spring force of the contact cage. As a consequence, the cable construction is deformed at the place where the contact cage is located. This in turn causes an undesirable change of the characteristic wave resistance of the cable at the contact cage. This change in wave resistance impairs the transmission quality of the high-frequency signals to be transmitted by the cable.
There is a need for a connection in which the cable shield can be connected to a connector without detracting from the electromagnetic shield effect and the electrical characteristics of the cable.